The above-referenced '814 patent describes a system for altering the content of an extrudate along the length of the extrudate using speed controlled gear pumps (sometimes referred to herein as “melt pumps”) to meter melt flowing in two or more extrusion lines that converge in a single die as schematically shown at 31 in FIGS. 1 and 2.
The expressions “alternate polymer extrusion,” “alternate polymer extrusion system,” and the like are not intended to limit the invention described herein solely to polymers, but to refer to extrusion of two or more materials that may include, for example, rubber or another material. Also, alternate polymers and polymers A and B as referred to herein may refer to the same polymers, but having, e.g., different durometers, colors, or other properties.
The aforementioned '814 patent relates to an alternate polymer extrusion system by which is meant a system that extrudes an extrudate that varies in content along its length. This can be, for example, a coextrusion of discrete layers varying in thickness or an extrudate that changes over from one material to another and back to the original material. The basic concepts of the '814 patent are completely validated. However, when the concepts of that '814 patent were extended to different sizes and types of tubing and different polymers, several problems arose. The use of melt pumps pursuant to my '814 patent is by far the most promising way to make alternate polymer tubing, profiles, sheet, etc., which change from one polymer to another repeatedly along their length. As explained in the '814 patent, other methods can alternate the constituents and make certain configurations, but do not come close to what the patented system can do. Several rather significant additions to the basic system are being added, as described here, to make a versatile system for producing high quality tubing.
In the system to which this invention relates, two or more extruders are used. An extruder is very good at many things:
(a) It can convey plastic pellets into a melting zone.
(b) It can melt them by adding frictional heat.
(c) It can mix various components quite well.
(d) It can generate the pressure to force plastic through a die.
What it cannot do is meter the plastic or other material accurately. The output of an extruder is heavily dependent on the back pressure. The output of an extruder will vary about 10% for every 1000 PSI variation in the pressure which the screw sees at the output. Further, when the extruder is used in a coextrusion set-up, where two or more extruders feed the same die, with each output forming one layer of a tube, sheet, profile, or the like, the output of each extruder is affected not only by the pressure caused by that particular extruder, but also by pressure changes in all the other extruders on the same die. In other words, the extruders react to each other. A further problem is that an extruder is not a positive shut-off device. There is always an open channel in the screw that moves melt in the extruder, and the extruder can “drool” even when the screw is not turning.
These conditions strongly militate against using extruders alone in an alternate polymer system. If one extruder is ramped up and the other down, the net result will not be proportional to the sum of the RPM of the two screws. The output of each extruder will be highly non-linear.
In the '814 patent, therefore, one uses a gear pump, or melt pump, on the output of each extruder. A gear pump is essentially a series of measuring cups on a wheel. The plastic fills the gear teeth at the input, travels around to the output side, where it finds that it cannot go through the tightly meshed gears, and so has no choice but to go out of the output. The output is highly precise. When the Harrel differential pressure control described in U.S. Pat. No. 4,209,476 is used around the pumps in an alternate polymer system, stability of output is typically 0.1%, and it is almost completely independent of what is happening in the other extruders. The U.S. Pat. No. 4,209,476 entitled Method and Apparatus for Fabrication of Extruded Articles, issued Jun. 24, 1980 (the '476 patent) is incorporated herein by reference. In the steady state, the extruders can be ramped up and down, and the output will be almost exactly proportional to the sum of the RPM's of the two melt pumps. One can thus ramp the speed of one up and the other down in such a way that the sum of the two RPM's stays the same, and, in the steady state, the total volume in the output will stay the same.
Correction for Differing Constituent Characteristics
One might suppose that in a tubing line, for example, if the volumes of polymer A and polymer B are the same, the dimensions of the tube the line forms would also be the same. Unfortunately, this is only true in the case where the two polymers have identical durometers, elasticity, and other physical characteristics. If two different polymers are extruded under identical operating conditions, the tubes the two make will differ in dimensions. The same is true for profile, sheet, or other extrusions.
The Harrel TUBETROL® extrusion control system provides a means for control of both ID and OD of tubing. The Harrel TUBETROL® extrusion control system and other Harrel products referred to herein are available from Harrel, Incorporated, 16 Fitch St., East Norwalk, Conn. 06855. The Harrel SHEETROL, PROFILE-TROL, extrusion control systems and other control systems can control the dimensions of other shapes. The Harrel BUMP TUBETROL extrusion control system is used to control operating parameters of the line such that the OD and ID of the tubing varies (or is sustained) along the length in a controlled manner. It is clearly possible to use these same techniques, not to create a variation in OD and ID, but rather to prevent it. The Harrel BUMP TUBETROL control system's techniques can also be used to produce dimensional variations along the length of the tube independent of the alternate polymer effects.
It is to be expected, therefore, that as the change is made from polymer A to polymer B in an alternate polymer extrusion, the Harrel BUMP TUBETROL control system's techniques will have to be used to compensate for differences in tubing dimensions caused by the change of material. This was perceived previously and is discussed in the '814 patent.
Compensation for Die Layer Drool
What was not considered in the '814 patent was the fact that there would still be significant “drool” from the output of each layer of the coextrusion die. Although drool is considerably improved by the use of the gear pumps, there is a significant volume of melt in the channels from the output of the melt pumps, where positive shutoff occurs, to the output of the die. In a typical system, an adapter downstream of the melt pumps is followed by a clamp from which the melt path leads to the die. The melt resident in the adapter from the melt pump to the clamp, and in the paths from the clamp to the die, plus the melt in the die channel itself from input of the die for that particular layer, through to the output of the die, must be kept to a minimum. It can never, however, be zero, and if nothing is done, that volume will “drool” out into the melt stream even when the melt pump for that layer is stopped.
Using a commercially available Genca Tri-die in an alternate polymer system to extrude tubing, the transition between polymer A and polymer B stretched over many inches of the tubing. This is unsatisfactory for applications of alternate polymer tubing in which one of the chief qualities being sought in the tubing was as short as possible a transition from one polymer to another of perhaps two to four inches. There was thus a need to reduce the drool of polymer from the melt path downstream of a gear pump when the gear pump slows or stops.